I'm replacing my thermostats with Honeywell Chronotherm IV Model 8600 (power stealing) and having some trouble with wiring. I have five thermostat wires coming out of the wall - red, white, green, blue and yellow. I have a 24 volt system with gas forced air and central a/c. All components are made by Goodman. Based on advice from a friend of my neighbor's I wired the Chronotherms as follows... jumped the Y and W terminals on the thermostat, attached the red wire to the R terminal, the green wire to the G terminal, the yellow wire to the Y terminal and the white wire to the B terminal. The blue wire is not connected. The thermostat powers up, but when I turned on the a/c, the air handler and the a/c unit started up, but the thermostat went blank and I cannot get the air hander and a/c unit to shut off unless I remove the thermostat from it's base mounted to the wall. I'd appreciate any advice anyone could provide. I'm hoping it's a simple wiring error. Thanks in advance for your help.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. It may be wired to have the control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan.

Got to run, you do need the blue wire to the thermostat.

Thanks for the reply... can you advise where the blue wire should be connected? The thermostat is currently wired as follows: white wire connected to the O terminal, green to G, yellow to Y and red to R. I have three remaining open terminals on the thermostat - B, W and RC. The R and RC terminals are jumped together and the Y and W are jumped together. I'm wondering if the current wiring scheme I have is correct less the blue wire and if so, where the blue wire should go.

Thanks for your help...

Does the thermostat have the red and blue wires coming off it?
The AC comes on because the white and yellow are jumpered. Only connect each to the W or Y. Leave the R and RC jumpered. Connect the blue to B. The thermostat should draw power from the transformer through the RC and the B connections.

There are no wires coming off the thermostat - only terminals to which wires from the wall should be connected. Based on your last post, I should remove the jumper between W and Y and connect the blue wire to the B terminal. Is this correct?

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Originally Posted by labman

Does the thermostat have the red and blue wires coming off it?
The AC comes on because the white and yellow are jumpered. Only connect each to the W or Y. Leave the R and RC jumpered. Connect the blue to B. The thermostat should draw power from the transformer through the RC and the B connections.

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Oops! My last answer should have started ''Does the TRANSFORMER have a red and a blue wire coming off it?''. Usually the red wire goes to the R or RC terminals on the stat where it is switched to what ever the stat calls for. The blue wire will complete the circuits inside the furnace except for running out to the AC relay. Sometimes that circuit will be wired up with any pair of wires. If the stat needs power, then both wires from the transformer must be run to it, connecting red to the RC and blue to the B terminals. Leave the red connected to the R and RC. Do not jumper the W and Y, but connect them to the white or yellow wire.

I ran the blue wire to the B terminal, removed the jumper between the Y and W and left the remaining wires as is - Red to R, Green to G and White to O. When I replaced the thermostat to the wall, it came on for a couple of seconds and then went blank. I checked the air handler in the attic and found that the fuse was blown. Any ideas? :confused:

Some furnaces use different windings for heating and cooling. When you had the W and Y jumpered, you may have had both windings on at the same time drawing too much current. Replace the fuse, and try again. You should still need the R and RC jumpered.

I tried it again without the Y and W terminals jumped and blew another fuse. The instructions indicate that this model thermostat is a 'power stealing' type. Does this mean it should be wired any differently?

I am not familiar with a power stealing thermostat. Conventional thermostats do not use any power, only switching one leg of the circuit as needed for heat, cool, and fan only. The programmable or other ones with a digital read out need a return connection for the other leg of the circuit. As long as the stat is calling for something, it could power its self on the current flowing to which ever. Once it was satisfied, and shut the circuit off, it would lose its connection to the return leg.

Does the transformer lose power when the fuse blows? Will the stat power up with the red and blue connected? Try connecting the red to R, and the blue to B. If the stat doesn't come on, jumper the R to RC. Then see if W, Y, and G are hot when it calls for heat, cool, and fan only. See if the white goes to the gas valve, the yellow to the AC relay, and the green to the fan. They may go to a terminal strip at the furnace labeled W, Y, and G. Then connect the white, yellow, and green one at a time with the power off. After each one, turn the power on and see if the fuse blows. You may have damaged the blower motor when you had the yellow and white jumpered.

I don't understand how it could work without the blue, but you might try it without it and without the white and yellow jumpered.

Can a ac unit be controlled by 2 thermostats ,1 is manual and the other is Digital ?